Studies of myocardial integrity which combine the use of metabolic substrates labeled with physiological positron-emitting radionuclides (carbon-11, oxygen-15, nitrogen-13) and positron emission tomography (PET) require fast data acquisition to permit "functional" imaging of physiological phenomena which change rapidly with time. Presently available PET systems are limited in their capacity for fast studies, not by radiation safety considerations, but by the fact that increasing the counting rate results in an increase in the number of random coincidences. On the basis of pilot experimentation carried out in our laboratory, we propose to design and build a PET system with faster data acquisition capabilities and a better signal-to-noise ration than presently achievable. The proposed system exhibits two unique features: (1) its scintillation detectors are fitted with cesium fluoride crystals, and (2) the image reconstruction process utilizes time-of-flight information in the spatial localization of annihilation events. The short half life of cesium fluoride (approximately 5 nanoseconds) will allow the system to handle counting rates ten times greater than are possible with sodium iodide or busmuth germanate detectors. The incorporation of time-of-flight information into the reconstruction process increases the signal-to-noise ratio in the image-forming data. Our main goal in the development of this system is to utilize it in ongoing studies of myocardial integrity which are limited by the performance of the imaging system now in use. The proposed system can also be employed in rapid functional studies in the brain and othe parts of the body.